Field of the Invention
The present invention relates to a zoom lens which is suitably used for an image projection apparatus that enlarges and projects an image.
Description of the Related Art
Recently, a zoom lens having a high resolution is required according to requirements of a high reality and high presence resulting from a super resolution of an image. Furthermore, for the zoom lens, an image plane with a higher flatness is required due to a decrease of a permissible width of depth by enhancing its resolution.
In an image projection apparatus, it is necessary to provide a space to guide illumination light to an image formation element, and accordingly a back focus of a zoom lens needs to be long to some extent. In the image projection apparatus, a pupil is determined by an illumination optical system. In order to ensure a satisfactory illuminance distribution, the zoom lens needs to be telecentric at a reduction conjugate side. It is preferred that various aberrations such as distortion and chromatic aberration of magnification are appropriately corrected so that distortion and color shift do not occur in a projected image with respect to an original image.
Japanese Patent No. 5053694 discloses a zoom lens which includes, in order from an enlargement conjugate side to a reduction conjugate side, lens units having negative, positive, positive, positive, negative, positive, and positive refractive powers.
However, in the configuration disclosed in Japanese Patent No. 5053694, changes of brightness, spherical aberration, astigmatism, and distortion during zooming cannot be effectively reduced, and accordingly it is difficult to achieve a higher resolution.